1. Field of the Invention
The invention relates to paper machine clothing useful for fabrication of dryer belts, employed in the dryer section of a papermaking machine, wet belts employed in the press section of such machines and forming wires which may be used on fourdrinier and cylinder machines and more particularly relates to such fabrics made from monofilaments of synthetic polymer resins.
2. Brief Description of the Prior Art
Papermaking machines are well known in the art. The modern papermaking machine is in essence a device for removing water from the paper furnish. The water is removed sequentially in three stages or sections of the machine. In the first or forming section, the furnish is deposited on a moving forming wire and water drained through the wire to leave a paper sheet or web having a solids content of circa 18 to 25 percent by weight. The formed web is carried into a wet press felt section and passed through one or more nip presses on a moving press felt to remove sufficient water to form a sheet having a solids content of 36 to 44 percent by weight. This sheet is transferred to the dryer section of the papermaking machine where dryer felts press the paper sheet to hot steam heated cylinders to obtain a 92 to 96 percent solids content.
On papermaking machines, endless belts are employed in the various sections to carry the sheet or web of paper. There are a wide variety of forms of the endless belts, some fabricated from metal and others from textile material such as cotton, cotton and asbestos or cotton, asbestos and synthetic fibrous or filamentous materials. The selection of a given material is dependent to some degree upon the use to which the fabric will be put, i.e.; as a forming fabric, dryer felt, etc.
One form of belt which has been used extensively as a forming wire in the forming section of the papermaking machine is one fabricated from an open weave of synthetic, polymeric resin monofilaments. Such fabrics generally perform well in the forming section although there are certain limitations. For example, the relatively open weaves, particularly when run at highest speeds, lack dimensional stability. This shortens the overal life of the forming wire which is subject to abrasion as it shifts in position on the machine.
Dryer belts for use in the drying section of the papermaking machine have historically been fabricated from dryer felt fabrics. In recent years, one form of belt commonly employed in the dryer section of a papermaking machine is referred to as a "screen" and is fabricated by weaving synthetic monofilaments or twisted multi-filaments together in an open weave. Although not subjected to any form of milling, and therefore not "felts" in the original sense of the term, these screen fabrics have also become known as "dryer felts". The endless belts are generally woven flat and the ends thereafter joined to form an endless belt. The weave selected may be a two or three layer weave of synthetic yarns such as multifilament, spun or monofilament yarns.
In carrying the formed paper web through the dryer section of the papermaking machine, the felt aids in drying, controls shrinkage of the paper web and prevents cockles. The felt fabric must possess strength, dimensional stability, resistance to chemical and thermal degradation, resistance to abrasion and have a functional permeability. In recent years all monofilament structured fabrics have been developed to meet the above-described needs of a dryer felt. However, dryer felts fabricated from all monofilament fabrics, have heretofore not been entirely satisfactory. Generally, the previously used fabrics have had relatively rough surfaces, which are brought in contact with the paper sheet to be dried. This rough surface can mark the paper sheet. This of course may be highly undesirable, resulting in a high percentage of unsatisfactory paper product.
It will also be appreciated that the screen type of "dryer felt" fabric is relatively open in design, resulting in a relatively high fabric permeability, i.e.; ;air permeability on the order of from about 70 to 700 CFM/sq. ft. at 1/2" water. Such fabrics advantageously permit free vapor passage through the fabric during operation of the papermaking machine. The high permeability of the fabric can be controlled by the incorporation of stuffer picks, as is well-known in the art; see for example British Pat. No. 1,207,446. However, the open design also means that the fabric may have lower dimensional stability, even if stuffer picks are included. This can be a problem, affecting the quality of paper being manufactured on the machine and the life of the dryer felt.
In any position on the papermaking machine, belts fabricated from monofilaments of synthetic polymeric resins are subject to abrasion and moist heat hydrolysis. In many of the prior art constructions the load bearing machine direction monofilament yarns may be rapidly degraded under some conditions of use so that the life of the dryer felt is shortened.
With the structured fabrics of the present invention, many of the above-described shortcomings of the prior art are removed. Dryer belts constructed according to the invention may be fabricated from an all monofilament fabric which provides an exceptionally smooth surface to contact the paper sheet. As a result, relatively mark free paper product is obtained, while all of the desired advantages of an all monofilament dryer felt are retained. The monofilament machine direction yarns employed in the fabrics of the invention are more resistant to degradative elements. The overall operating life of the forming wires and felts is significantly increased over prior art felts.
It will be appreciated that there is an extensive range of prior art descriptions in the field of papermaker's fabrics. Representative of such descriptions are those found in U.S. Pat. Nos. 2,260,940; 2,354,435; 2,748,445; 3,060,547; 3,158,984; and British Pat. No. 980,288.